Strong anharmonicity dictates ultralow thermal conductivities of type-I clathrates

Abstract

Type-I clathrate solids have attracted significant interest due to their ultralow thermal conductivities and subsequent promise for thermoelectric applications, yet the mechanisms underlying these properties are not well understood. Here, we extend the framework of vibrational dynamical mean-field theory (VDMFT) to calculate temperature-dependent thermal transport properties of $X_8$Ga${}_{16}$Ge${}_{30}$, where $X=$ Ba, Sr, using a many-body Green's function approach. We find that nonresonant scattering between cage acoustic modes and rattling modes leads to a reduction of acoustic phonon lifetimes and thus thermal conductivities. Moreover, we find that the moderate temperature dependence of conductivities above 300 K, which is consistent with experimental measurements, cannot be reproduced by standard perturbation theory calculations, which predict a $T^{-1}$ dependence. Therefore, we conclude that nonperturbative anharmonic effects, including four- and higher-phonon scattering processes, are responsible for the ultralow thermal conductivities of type-I clathrates.

17 pages, 10 figures

References in corpus (5)

• Impact of Rattlers on Thermal Conductivity of a Thermoelectric Clathrate: A First-Principles Study
• First-Principles Lattice Dynamics Method for Strongly Anharmonic Crystals
• Conductivity of an electron coupled to anharmonic phonons
• Anharmonic lattice dynamics and thermal transport in type-I inorganic clathrates
• Occupational Disorder as the Origin of Flattening of the Acoustic Phonon Branches in the Clathrate Ba${}_8$Ga${}_{16}$Ge${}_{30}$

Cited by in corpus (1)

• Generalized quantum master equation from memory kernel coupling theory